There is an increasing demand for cryogenic air separation plants to produce very large quantities of oxygen for use for example in direct reduction steel making processes, coal gasification processes, and partial oxidation processes in which natural gas is converted to synthetic gas.
Most modern commercial air separation plants employ a higher pressure rectification column having its upper end in heat exchange relationship with the lower end of the lower pressure rectification column. Cold compressed air is separated into oxygen-enriched and nitrogen-enriched liquids in the higher pressure column, and these liquids are transferred to the lower pressure column for separation into nitrogen-rich and oxygen-rich products. Large quantities of energy are required to compress the feed air. U.S. Pat. No. 3,731,495 discloses a process for reducing the external power consumption of the process. The process employs a nitrogen-quenched power turbine. A portion of the compressed feed air is mixed with fuel and combusted. A hot combustion mixture is then quenched with waste nitrogen-rich gas from the lower pressure rectification column and the resulting gaseous mixture is expanded in a power turbine. The expansion provides energy to compress the feed air to the system. A major disadvantage of this process is that the pressure of the gaseous mixture expanded in the power turbine can be no higher than that of the waste nitrogen mixed with the combustion gases. As pointed out in U.S. Pat. No. 4,224,045, commercially available power turbines have optimum inlet pressures in excess of the optimum operating pressure of the lower pressure rectification column. Accordingly, U.S. Pat. No. 4,224,045 proposes compressing the waste nitrogen prior to using it to quench the combustion mixture.
In both these U.S. patent specifications, the turbine is employed primarily to produce a quantity of external work which is sufficient to meet the requirement of the air compressor. However, when large quantities of oxygen are required for processes such as direct reduction steel making or coal gasification it is desirable to use the turbine to generate a quantity of electricity greatly in excess of the demands of the air compressor. Accordingly the air compressor feeds both the air separation plant and the turbine. The air so supplied to the turbine is typically used to support the combustion of fuel gas from the gasifier or blast furnace. In the process disclosed in U.S. Pat. No. 4,224,045 the pressure at which the air feed compressor operates is substantially the same as that of the operating pressure of the higher pressure rectification column and is selected so as to maximise the efficiency of the higher pressure column. However, in the kind of process described above where the air also feeds the combustion chamber of the power station, the pressure to which the air is compressed will be governed by the inlet pressure selected for the turbine. Typically, such turbines are operated at relatively high pressures above the optimum operating pressure of the high pressure column in a conventional double rectification apparatus. Indeed, with the upper end of the higher pressure column in heat exchange relationship with the lower end of the lower pressure column through a condenser reboiler, selecting the pressure for the higher pressure column effectively dictates what the pressure is in the lower pressure column and what the reflux ratios are in both columns. This inflexibility makes it difficult to achieve efficient operation of the two columns.
In U.S. Pat. No. 4,655,809 there is disclosed a process in which the oxygen and waste nitrogen streams are generated by an air separation apparatus including only one rectification column. In order to provide reflux and reboil for the rectification column it is necessary to operate a heat pump cycle. However, to operate the rectification column at substantially the same pressure as that to which the incoming air is compressed while producing nitrogen at a high pressure, would require the generation of very high pressures in the heat pump circuit. Accordingly, in these circumstances, the heat pump circuit would have a very high power demand and renders the air separation process relatively inefficient.
There is therefore a need for an improved air separation process and apparatus which are capable of operation with a relatively high pressure air feed, which are more flexible in operation that a conventional double column, and which have a reduced requirement for heat pumping work in comparison with a single column.